Over the past several decades automotive vehicle manufacturers have satisfied continually-decreasing limits on the amounts of carbon monoxide, unburned hydrocarbons, and nitrogen oxides (largely NO, with smaller amounts of NO2, collectively, NOx) that are discharged to the atmosphere in the exhaust from vehicle engines. These requirements of reduced exhaust emissions are combined with requirements for increased fuel economy. These combined requirements have required ever more sophisticated engines, computer control of engines, and exhaust gas aftertreatment systems, including catalytic converters for oxidation and reduction reactions, in the exhaust stream.
Present exhaust aftertreatment systems are quite effective in treating the exhaust from a warmed-up engine because the catalyst materials have been heated to temperatures (e.g., 200-250° C. and above) at which they serve to effectively oxidize carbon monoxide and residual hydrocarbons to carbon dioxide and water, and to reduce nitrogen oxides to nitrogen. These aftertreatment systems have been quite effective for both gasoline-fueled engines operating at a stoichiometric air-to-fuel mass ratio (about 14.7:1) and diesel engines (and other lean-burn engines) which operate with considerable excess air (e.g., air to fuel mass ratio up to about 25:1). However, it has been difficult to treat exhaust emissions immediately following a cold engine start, before the exhaust has heated a catalytic converter to its effective operating temperatures for the catalytic reactions. It is realized that such untreated emissions will make-up a significant portion of the total emissions at the tailpipe in the mandated testing of engine emissions control systems. The problem is particularly difficult with the treatment of mixed nitrogen oxides (NOx) in the exhaust of diesel engines. There is, therefore, a need for better systems for treating the exhaust gas from an engine following a cold start. The need is particularly acute in lean-burn engines, such as diesel engines, which tend to produce cooler exhaust streams because of the excess air used in the combustion mixtures charged to their cylinders.